The need for higher precision resistors in integrated circuits is increasing due to the demand for higher precision measurement and instrumentation circuitry. Additionally, many integrated circuits that employ both analog and digital circuit components together require close matching of resistors. Some of these cases also require that the resistors to be matched have high resistance values. The two requirements of precision and high resistance values are especially difficult to achieve simultaneously when only a small area is available in the semiconductor wafer.
Resistors can be implemented in several ways in integrated circuits. They can be constructed using metal, polysilicon, n+ diffusions, p+ diffusions or tub regions. For metal and polysilicon, the sheet resistance is typically low and adequate overall resistance matching may be achieved for lower values of resistance. However, adequate matching of larger resistance requires the use of a large area in the semiconductor wafer. For tub regions, the sheet resistance is typically high. Resistance matching is difficult, however, due to the fact that the sheet resistance is dependent on the value of tub to substrate voltage. For n+ diffusions and p+ diffusions, matching properties are typically intermediate to the cases discussed above, with high precision and large resistance values still being difficult to achieve. An additional problem arises in that only some of the resistance formulation techniques may be practical in any given semiconductor production process.
Semiconductor wafer "real estate" is of prime concern in the design and production of semiconductor circuits. As CMOS technology continues to allow gate sizes to shrink, the potential for an increase in circuit density and therefore the total number of semiconductor devices allowed per semiconductor circuit chip has increased dramatically. In devices and circuits where high component precision is usually not required, such as digital circuits, resistor area size is more easily managed. However, in many analog devices where resistor precision, large resistance values and even resistance matching are required, the resistor area required may be the controlling factor in determining device density. This situation may significantly reduce the device density otherwise achievable, and even make a device design marginally economical from a market perspective.
Accordingly, what is needed in the art is a way to readily provide small area, precision resistors that allow adequate resistance matching even for large resistance values.